1. Field of the Invention
The present invention generally relates to magnetic recording media suited for high-density recording, magnetic storage apparatuses using such magnetic recording media and methods of producing such magnetic recording media, and more particularly to a magnetic recording medium which has a recording layer formed by two magnetic layers which are antiferromagnetically exchange-coupled, a magnetic storage apparatus using such a magnetic recording medium and a method of producing such a magnetic recording medium.
2. Description of the Related Art
Recently, the recording densities of magnetic recording media have increased dramatically, even at a rate reaching 100% per year. However, in the popularly employed longitudinal (or in-plane) recording system, it is expected that a limit of the longitudinal recording density will be on the order of 100 Gb/in2 to 200 Gb/in2, because the medium noise will increase and the output will decrease in the high-density recording region, thereby deteriorating the signal-to-noise (S/N) ratio. In addition, in the longitudinal recording system, the thermal stability of the magnetic recording medium becomes a problem due to the self-diamagnetic field with respect to written bits and the diamagnetic field from the adjacent bits which increase for the high-density recording.
In order to reduce the medium noise in the high-density recording region, it is necessary to reduce the size of the magnetic grains, that is, the crystal grains forming the recording layer, and to reduce the magnetic interaction between the magnetic grains. When the size of crystal grain forming the magnetization unit is reduced, it is possible to reduce the zigzag of the boundary between the magnetization units, that is, the magnetization transition region. There are several known methods of reducing the crystal grain size, such as adding Ta, Nb, B, P and the like to a CoCr alloy forming the recording layer. Moreover, there is a known method which reduces the size of the crystal grains forming an underlayer of the recording layer so as to reduce the size of the magnetic grains grown epitaxially on it, Yu et al., “High-Density Longitudinal Recording Media With CrMoB Underlayer”, IEEE Trans. Magn., vol. 39, No. 5, September 2003, pp. 2261-2263 proposes reducing the thickness of the underlayer and adding B to the underlayer in order to reduce the crystal grain size of the underlayer.
Furthermore, as a method of reducing the magnetic interaction between the magnetic grains, it is known to be effective to increase the Cr-content or B-content of the CoCr alloy forming the recording layer, so as to promote segregation and isolate the magnetic grains by the grain boundaries between the formed magnetic grains.
On the other hand, in the longitudinal magnetic recording medium, it is known that an isolated wave peak width at half-value PW50 of the reproduced output waveform can be described by the following formula (1) of relationships of static magnetic characteristics of the magnetic recording medium, where Hc denotes a coercivity, Mr denotes a remanent magnetization, t denotes a thickness of the recording layer, a is proportional to (t×Mr/Hc)1/2, and d denotes a magnetic spacing.PW50=[2(a+d)2+(a/2)2]1/2   (1)
It may be seen from the formula (1) that in order to reduce the isolated wave peak width at half-value PW50, the coercivity Hc should be increased and the thickness t should be reduced. Since the resolution of the reproduced signal improves as the isolated wave peak width at half-value PW50 becomes smaller, it is desirable to make the thickness t of the recording layer small and to make the coercivity Hc of the recording layer high, in order to realize the high-density recording.
However, when the reduction of the magnetic grain size and the reduction of the magnetic interaction are promoted, the diamagnetic field increases as the recording density increases, thereby making the problem of the thermal stability even more serious. As a countermeasure, an anisotropic field Hk of the recording layer is conventionally increased. But when the anisotropic field Hk is increased, it becomes more difficult to record information on the recording layer. Consequently, the recording magnetic field needs to be increased, and it becomes essential to develop a magnetic pole material having a high saturation magnetic flux density for the recording head element. The development of the magnetic pole material must not only satisfy the high saturation magnetic flux density, but must also simultaneously satisfy various required characteristics, thereby making the development extremely difficult.
A Japanese Laid-Open Patent Application No. 2001-056924 proposes a magnetic recording medium which simultaneously realizes reduced medium noise and thermal stability. This so-called Synthetic Ferrimagnetic Medium (SFM) has a recording layer that is formed by two antiferromagnetically exchange-coupled magnetic layers. In the SFM, the volume of the magnetic grains substantially becomes a sum of volumes of the magnetic grains of the two antiferromagnetically exchange-coupled magnetic layers. For this reason, the thermal stability is greatly improved, and the magnetic grain size can be reduced. As a result, the SFM can realize improved thermal stability, reduced medium noise and high S/N ratio.
However, when the thickness of the magnetic layer of the SFM is reduced as described above in order to improve the resolution, a coercivity squareness ratio decreases to thereby deteriorate the resolution instead of improving the resolution. In addition, a ratio Siso/Nm of an isolated wave average signal output Siso and a medium noise Nm, and a ratio S/Nm of an average output S at a maximum recording density used and the medium noise Nm, deteriorate.
Moreover, in a case where an exchange coupling energy acting between the two magnetic layers is excessively large, the thermal stability improves, but recording characteristics such as an overwrite performance and a Non-Linear Transition Shift (NLTS) deteriorate. Although there is a method of increasing the recording magnetic field to suppress the deterioration of the recording characteristics, it is difficult to increase the recording magnetic field as described above.